In a large (6-quart) pot, heat the butter on medium high heat until it melts and foams up. Continue to heat until the foam subsides a little and the butter just begins to brown.

Immediately toss in the sliced leeks and onions. Stir to coat with the butter. Cook for several minutes, reducing the heat to medium if necessary, until the leeks and onions are translucent and wilted.

Add the chopped potatoes, salt, and water or stock. Bring to a boil, reduce to a simmer. Cook, partially covered for 30-40 minutes, until the potatoes are completely cooked through. Remove from heat.

Purée using an immersion blender or working in batches, blend in a blender. (Careful! With hot liquids only fill the blender 1/3 of the way full, and hold the blender top on with your hand while blending.)

Purée until completely smooth. If you want an even smoother soup, you can take the extra step of pressing the purée through a sieve with a rubber spatula.

Allow to cool a bit before stirring in the sour cream and whipped cream. Allow to cool completely and chill in the refrigerator. The soup should be served just below room temperature (maybe 65°F or 18°C). If it is too cold, it won’t taste as good.